Heart rate drops from 160 to 100 after 50 min running

Sometimes it happens the same to me, after about 10 minutes the bpm increases suddenly from 130 to 150... I guess it's a issue form the watch readings.

It is really sad we can't trust the readings... because if you look at the picture I uploaded, my hear rate could have been anywhere in between, but funny enough it stayed around 100 until the end of the workout.

The only scenery when the optical heart rate sensor accurate is, at rest. Is good enough for stress measurement etc.

For accurate training analysis  a heart rate strap recommended.

It is not so strange that it acts like that. The sensor picks up a noisy signal. The algorithms in the watch will continuously try to determine which of the many frequencies that is your heart rate. Some of the frequencies it will see are from your movements, like your running cadence. To avoid mixing them up, it tries to filter out that frequency. It is very possible that you got close to that, and the watch found that it had been wrong, and the heart rate is one of the other ones. It also knows that sudden changes is heart rate is unlikely, so once it locks on to a wrong frequency, it is likely to keep tracking it.

Interesting. I thought many of the signals would come from different sensors such as accelerometer for the cadence, vertical oscillation etc, whilst heart rate would go through the leds. 
Thanks for the input.

Sometimes I think Garmin employees are writing here pretending to be users. No, it's not normal for your heart rate to suddenly drop by 60 bpm after 50 minutes of running. Please stop framing this as normal. These sports watches are made precisely for this purpose, are expensive and do not fulfil their advertised promises. It gets worse with every firmware update. OP didn't even run intervals but an absolutely even pace. Why are these watches even called "Forerunners" if it "normal" that they are not able to read your heartrate? Why did it work in the past and why don't Apple or Huawei have such problems?

I must agree with you. I am a former Forerunner 235 owner since 2018 (so 7 years) and never had that before. I was happy to upgrade to have more data available, but now I wonder... I don't know about other brands, but I confess I am a bit disppointed with Garmin on this one.

I have no connections to Garmin, but I am a robotics engineer, so I have an understanding about how the technology works. I have had watches from other brands, like Polar and Fitbit, and they had the same misreadings. Most of the time it works fine, but sometimes the readings are wrong. If you need reliable data, you should use a chest strap.

6742038 said:

Why did it work in the past and why don't Apple or Huawei have such problems?

Problems like that one? OHRS, by design, is not that precise. And, in the long run, the blood goes down to the legs, making readings more difficult. I have now and then reading artifacts too, but on the last betas, it goes better IMHO.